<I really like this idea of teaching momentum before force.
Of course you don't *need* a justification for momentum, if you are
willing to tell the students "this is/will be useful and important later;
trust me". But I think you indicated previously that you like to organize
class so that the concepts introduced are all motivated. (Which also seems
like a very good idea.)
So the question is, how do you motivate the students to focus on the
combination m*v, without reference to force? The more I think about it,
the more I think the answer may be conservation of momentum.>
James,
I misunderstood your question. Here's how I approach conservation of
momentum. If you have other ideas, I'm all ears. Actually I'm all ears
anyway. My sisters used to tell me my ears looked liked two opened cab
doors. Since then, I wear my hair longer. I posted this message back in
March, so for those that read it then this isn't deja vu.
I like to refer to the felt picture of the dogs playing cards. Two dogs,
Fido and Rex are in a closed room. No money can come in or leave the room.
Fido has twenty-five dollars and Rex has thirty dollars. What is the total
amount of money in the room? If Fido loses ten dollars, how much money does
Rex gain? If Rex loses five dollars/hour, then how many dollars does Fido
gain (lose) per hour? Plot Rex's dollars, Fido's dollars, and the total
dollars in the room versus time. How do you compare Fido's and Rex's
dollars? How do the slopes compare? How do the changes in dollars compare?
Almost all students seem to get this graph.
After all this, I define momentum, impulse and force (in terms of a
momentum-time graph). Then I use video analysis to analyze various
collisions and look at the graphs showing the relationship to the card game.
A video analysis example of a two dimensional collision is shown on my web
site listed below.